Dutch authorities unable to identify source of bluetongue strain

Bluetongue cases are surging yet again on the continent after the warm summer months brought conditions ideal for the Culicoides midge which spreads the disease.

The Netherlands had been free of the virus since 2009 but has recorded a steady increase in case numbers in recent weeks.

This is despite the country using emergency approval procedures to secure access to bluetongue vaccines.

The current wave of bluetongue in the Netherlands – bluetongue serotype 3 - is much more aggressive than the bluetongue-8 strain that struck the country between 2006 and 2008.

Vaccines which have been widely used by Dutch farmers are proving to be less protective than those which were used to fight this previous strain, according to preliminary evidence emerging from studies of on-farm use.

A follow-up shot delivered around four to five weeks after the initial vaccination is hoped to boost the new vaccine’s efficacy.

Maps published by the Dutch ministry of agriculture put the number of cases confirmed by lab testing at 5,318 this year up to 5 September. This figure is a 93% jump on the number of confirmed cases reported by the country’s authorities just three weeks previous.

Research from Wageningen University found that in addition to short-distance dispersal of the virus via infected midges, other transmission routes probably played a role in spreading the disease throughout the Netherlands in autumn 2023. Livestock movements have been identified as “one of the likely alternative transmission routes” to the midge vector.

Britain is also struggling to contain the virus, as the UK’s Department for Environment Food and Rural Affairs reported this weekend that 34 farms went down with bluetongue since 26 August.

Restricted zones are in place across Norfolk, Suffolk, Essex and parts of greater London, while a new 20km bluetongue control zone is in place around a farm in east Yorkshire after a case was detected there last week. East Yorkshire had previously been free of the disease.

Prior to the 26 August case, the UK had not recorded a confirmed case of the virus since March.

The entire island of Ireland remains free of bluetongue and, if a case was to emerge, the live export trade could take a significant hit as restrictions would be imposed.

Where did the new strain originate?

The main strain currently circulating in Britain and mainland Europe is a variant of bluetongue serotype-3, which was first detected on four farms in central Netherlands in September 2023.

Genetic analysis suggests that this variant is different to strains of bluetongue-3, which previously cropped up in Sicily, Israel and parts of Africa.

Wageningen University undertook testing of bulk milk tanks and determined that a new strain likely emerged around the location of these farms, but the exact geographical origin and route of its introduction remain unknown.

This retrospective screening of bulk milk samples in the month prior to these cases suggests that bluetongue had only recently started circulating in the affected area.

Bluetongue was detected in the Netherlands, Germany, Belgium, France, Luxembourg, Spain, Denmark and Britain in 2023.

Cases have been confirmed in the Netherlands, Germany, Belgium, Britain, France, Luxembourg and Denmark this year.

Wageningen’s bluetongue expert, Piet van Rijn, recently described the bluetongue-3 as being a strain “much more aggressive” than the one the country grappled with up to 2008.

“As the midges will remain active until late autumn, I don’t expect the virus to disappear this year and we should be prepared for its re-emergence next summer,” he said.

“Vaccination is our only remedy to combat this aggressive variant of bluetongue virus.”

Live exports from Britain

EU member states are hit with restrictions on live exports on the discovery of bluetongue and further restrictions are applied to semen exports from affected areas.

Ireland introduced a live export ban on Britain in November of last year. The Department of Agriculture stated that for the trade to recommence, UK authorities would need access to an EU-approved bluetongue-3 vaccine and/or establish EU-recognised disease-free zones in Great Britain.

The UK’s Department for Environment Food and Rural Affairs subsequently moved to approve three bluetongue-3 vaccines which will be permitted for use on the basis of regionally-targeted general licences or specific licences issued through an application process.

How does the current strain affect livestock?

Dutch animal health company Royal GD is currently conducting a study into bluetongue-3’s effects on cattle and sheep vaccinated against the virus.

It has so far found that the main symptoms cropping up on vaccinated dairy farms participating in the study have been a “significant drop” in milk production and fever, but not the severe lameness witnessed with bluetongue before vaccines were available.

Infected vaccinated cattle have been found to recover “relatively quickly” but animals are observed to go down with the disease “again and again”.

For sheep, while cases of severe lameness are prevalent in infected vaccinated animals as a result of red swelling on coronary bands, the characteristic swelling of tongues and symptoms visible around the mouth have not.

The research found lower mortality rates in vaccinated flocks in comparison to 2023’s outbreaks, in which only 25% of sheep survived infection.

Accurate detection

Further research from Wageningen noted that, during an outbreak of a disease, farmers and vets are more aware of the disease’s symptoms.

Should these clinical signs be misdiagnosed, authorities could face challenges in properly responding to a disease, this research warned.

Its authors referenced orf, pasteurellosis, strawberry footrot and photosensitisation as diseases whose symptoms can bear a resemblance to those of bluetongue. If these other diseases are wrongly diagnosed by farmers or vets, inaccuracies can result in the number of bluetongue cases notified and lab testing remains the “gold standard” for conclusively ruling whether or not symptoms are a sign of bluetongue infection, they said.

There are currently no vaccines effective against bluetongue-3 available on EU markets.

However authorities in an individual member state may choose to authorise the use of a vaccine in the case of an outbreak, as has occurred in the Netherlands and Germany.

The Department of Agriculture here has said that its current focus on bluetongue is to keep the disease out of Ireland, and on surveillance efforts to detect any cases which may emerge as early as possible.

The importation of infected animals poses one of the biggest risks to keeping bluetongue out of the country. Farmers should report any suspected cases immediately to their local Regional Veterinary Office. The disease’s symptoms include, but are not limited to:

Farmers have been advised by the Department to source livestock locally and to only import animals during the during the lower-risk December to March period, if they determine that they must source livestock from overseas. The Regional Veterinary Office should be contacted before any animals arrive on-farm from mainland Europe to arrange post-import blood testing.

Blood must be sampled within five days of an animal’s arrival to Ireland and a second sample will be taken 10 days after the first.